Rice Milling System

Rice Milling System

Nitat Tangpinijkul Post-harvest Engineering Research Group Agricultural Engineering Research Institute Department of Agriculture

Rice Milling The processes of converting paddy into rice • Removing the husk from the paddy • Removing all or part of the bran layer

The basic objective of a rice milling system is to remove the husk and the bran layers, and produce an edible, white rice kernel that is sufficiently milled and free of impurities. Depending on the requirements of the customer, the rice should have a minimum of broken kernels.

Factors Affecting Milling Recovery • • • •

Paddy quality Type of machinery, efficiency, system Operator Others • milling degree • ambient condition

Milling Recovery = Weight produced milled rice x 100% Weight processed paddy

Course Module 4 Lesson 1

: : :

Grain Quality Rice Milling Introduction to the milling process and its outputs

Good Equipment

Good Paddy Rice

If

Then

you have good quality paddy in a well-maintained mill that is operated by a skilled miller,

the mill will produce high quality head rice.

you use poor quality paddy,

the mill will always produce poor quality milled rice, despite the skill or the miller or maintenance of the mill.

the miller is not skilled,

the use of good milling equipment and good quality paddy does not guarantee a high quality product

Skilled Miller


: : :

Grain Quality Determining the Physical Characteristics of Paddy Rice The quality characteristics

Quality characteristics of paddy

These characteristics are determined by the environmental weather conditions during production, crop production practices, soil conditions, harvesting, and post harvest practices


: : :


QUALITY CHARACTERISTIC: MOISTURE CONTENT

Moisture content has a significant influence on all aspects of paddy quality. To obtain high yields, it is essential that paddy be milled at the proper moisture content. Paddy is at its optimum milling potential when its moisture content is 14%


: : :


QUALITY CHARACTERISTIC: IMMATURE

Immature rice kernels are very slender and chalky and result in the production of excessive bran, broken grains and brewer’s rice


: : :


QUALITY CHARACTERISTIC: VARIETAL PURITY

A mixture of varieties in a sample of paddy causes difficulties in milling and usually results in reduced milling capacity, excessive breakage, lower milled rice recovery, and reduced head rice yields.


: : :


QUALITY CHARACTERISTIC: DOCKAGE

Dockage includes chaff, stones, weed seeds, soil, rice straw, stalks and other foreign matter. These impurities generally come from the field or from the drying floor.


: : :


QUALITY CHARACTERISTIC: DISCOLORED

Water, insects and heat exposure can cause paddy to deteriorate through biochemical changes in the grain which may result in the development of offdoors and changes in physical appearance


: : :


QUALITY CHARACTERISTIC: CRACKED

Overexposure of mature paddy to fluctuating temperature and moisture conditions leads to the development of crack in individual kernels.

OTHERS QUALITY CHARACTERISTIC: • Weight per volume or density, gram/litre Weight per 100 or 1000 grains Thai rice variety: 100 grains weight 2.25 - 3.67 g • Color of Husk Affecting color of parboil rice • Color of Brown rice Affecting color of parboil rice Milling recovery and quality of milled rice grade

• Grain Dimension Grain size and shape (length-width ratio) is a varietal property. Long slender grains normally have greater breakage than short, bold grains and consequently have a lower milled rice recovery Type of Paddy : Extra long Long Medium Short Sub-type of Paddy Slender Bold Round

Length of brown rice ≥ 7.5 ≥ 6.5 < 7.5 ≥ 5.5 < 6.5 < 5.5 : Length/Width Ratio ≥ 3.0 ≥ 2.0 < 3.0 < 2.0

Thai rice variety:

Length 7.1 - 7.6 mm Slender 3.1 – 3.4

• Chalkiness • grain with opaque areas in the endosperm, caused by the loose packing of the starch and protein particles, breaks more readily during milling than clear grain, greatly reducing its market value • controlled by genetic and environment

white center

white belly

white back

Good Quality Paddy • • • • • • • •

uniformly mature kernels uniform size and shape (vareital purity) free of fissures and cracks free of empty or half filled grains less chalky, red kernel free of contaminants such as stones and weed seeds moisture content ~ 14% high milling yield variety

Ø Ø Ø Ø Ø Ø

Good variety, good crop management Harvest at optimum period Drying immediately after harvest to safe level m.c. Proper drying to prevent cracking Good aeration during storage to prevent grain deterioration Prevent birds & rodents during storage

Purity of paddy Milling Recovery = Weight produced milled rice x 100% Weight processed paddy Purity % 100 99 98 97 96 95

Impurity % 0 1 2 3 4 5

Milled rice % 68.00 67.32 66.54 65.96 65.28 64.60

Cracks Bran

Rice grain

Embryo

Bran Embryo

Rice flour Splinters

+

Total bran produced = when whitening cracked grains

Paddy Quality Good

Crack

Immatured

Japonica

Paddy

100.0

100.0

100.0

100.0

Husk

23.0

23.0

25.0

17.0

Brown Rice

77.0

77.0

75.0

83.0

Bran

8.0

10.0

10.0

7.5

Milled Rice

69.0

67.0

65.0

75.5

Brewer Rice

1.0

1.5

1.5

0.5

Recovery

68.0

65.5

63.5

75.0

Size of Rice Mill • Small

12 ton/day

• earning from bran, broken or money as milling wage • no grading of milled rice

• Medium 13-59 ton/day • commercial mill • for domestic market

• Large > 60 ton/day • for domestic market and export

Number of rice mill by region, 1993 - 1997 Year

North-

Northern

Eastern

Central

Southern

Plain

Whole Kingdom

1993

25,213

10,183

5,223

4,400

45,019

1994

24,905

10,104

5,156

4,395

44,560

1995

24,888

10,065

5,129

4,390

44,472

1996

24,887

10,049

5,114

4,386

44,436

1997

28,533

8,779

3,232

2,731

43,275

Source : Factory Control Division, Ministry of Industry

Number of rice mill in Suphanburi province (2004) Number of village Number of Rice mill < 5 ton/day > 5 < 20 ton/day > 20 ton/day

962 194 90 (46.4%) 42 (21.6%) 62 (32.0%)

PRICE LIST OF MODERN RICE MILL RICE ENGINEERING SUPPLY CO., LTD. (BANGKOK, THAILAND) For Month January-April 2008

Capacity (Paddy) Ton/Hour Ton/Day 2-3 4-5 8-10 18-20

50-80 100-120 200-240 420-500

Price (F.O.B) (USD) 348,000 624,000 1,050,000 1,824,000

All price exclude installation cost, electric motor, cable, control etc.

The Aim of Rice Milling • To attain the highest yield of white rice • With the best quality – Remove least amount of hull and bran – Minimum brokens – Little foreign matter

Rice milling system • 0ne step process • Two step process • Multi stage process In a one step milling process, husk and bran removal are done in one pass and milled or white rice is produced directly out of paddy. In a two step process, removing husk and removing bran are done separately, and brown rice is produced as an intermediate product. In multistage milling, rice will undergo a number of different processing steps.

Rice Milling Process Paddy into rice mill

Cleaning

Impurity

Paddy

Husking

Husk

Paddy, Brown rice Paddy

Paddy Separation Brown rice

Whitening & Polishing

Bran

Milled rice Head rice

Grading and Blending

Broken

Paddy Intake • Quality input = quality output • Garbage in = garbage out! The quality of milled white rice will be dependant on the quality of the paddy or rough rice coming into the mill as well as the milling process. Good paddy into the mill means quality milled rice output. Paddy intake chute

Paddy Cleaning • After harvest, foreign matter can be as high as 5-10% While most paddy has been cleaned after harvesting, some foreign material is still present. This can be as high as 5-10%.

Rice pre-cleaner

Pre-cleaning of Paddy • Remove foreign material such as sand, stones, straw, seeds etc. from paddy • Prevent damage to the milling machinery • Prevent clogging which cause the reduction in capacity and efficiency of rice mill • Remove materials that cause a reduction in the grade of milled rice

Paddy Precleaner • Sieve: separate large/ small impurities • Aspirator: separate light impurities • De-stoner: separate same size impurities but heavier • Trieur (Length Grader) : separate impurities having same width but different in length • Magnet : separate irons

Open double-sieve precleaner

Examples of perforated sheets and wire mesh

Closed-type single-action aspirator precleaner

Closed-type double-action aspirator precleaner

Double-drum type of Precleaner (West Germany)

Single-drum type of Precleaner (Japan)

Magnetic separator

Brass

Magnet

A, B: Simple permanent magnet C: Rotating brass cylinder and permanent magnet

De-stoner

Paddy in

Paddy

Air stream

Stone

Vibration direction Stone rice

Wind direction

De-husking • Underrun disc huller • Rubber roll huller • Centrifugal disc huller

Underrun disc huller Paddy

Brown rice Husk

Particulars of the underrunner disc huller: V = peripheral speed (recommended 14 m/s); W = width of coating; D = stone diameter; and W/D = 1/6 or 1/7.

Ratio of W/D ~ 1/6 or 1/7 Composition: • Emery grit 14, 16 • Silicium carbide grit 16 Peripheral speed ~ 14 m/s

Under-runner disc huller peripheral speed-curve for V = 14 m/s. D = stone diameter = 700 mm (0.7 m); and V peripheral speed = 14 m/s. V = ¶ x D x n (m/s) 60 n = 60 x V = 60 x 14 ¶xD 3.14 x 0.7 = 380 rpm.

Capacity and power requirement of underrun disc huller

Diameter (mm) 750 1000 1250 1400

Capacity (kg of paddy/hr) 450-600 700-1000 1000-1400 1600-2100

Motor (hp) 3 3.5 4 5.5

Underrun disc huller • Advantage § Operational simplicity, low running cost § The abrasive covering can be remade at the site • Disadvantage § Given grain breakage § The abrasions to outer bran layers

Rubber Roll Huller

Rice Huller

Rollers

Dehusking principle of rubber roll huller: • The rubber roll huller consists of two rubber rolls. • Both rolls have the same diameter. • One has a fixed position, the other is adjustable to obtain the desired clearance between the two rolls. • The rolls rotate in opposite directions. • The adjustable roll speed running about 25% slower than the fixed one.

Size and speed of the rubber roll When the rolls are new, their peripheral speed is about 14 m/s so that a smaller roll runs faster than a larger one.

Diameter

Width

High speed

Low speed

(mm)

(Inch)

(mm)

(Inch)

(rpm)

(rpm)

150

6

64

2.5

1320

900

220

8.5

76

3

1200

900

250

10

250

10

1000

740

The relation between paddy variety and hulling capacity • The wear for long grain variety is higher than short grain. • The wear of adjustable roll is lower than the fixed one.

Durability of Rubber Rolls Type (Inch)

Short grain (ton)

Long grain (ton)

10

300

170

8

240

140

6

110

60

4

75

40

3

35

20

2.5

30

15

Capacity and power requirement for different rubber roll hullers Size

Diameter

Width

Capacity (ton/hr)

Motor

(Inch)

(mm)

(mm)

Long grain

Short grain

(hp)

4

100

220

0.9

1.25

2.5

6

150

220

1.2

1.9

4.0

10

254

254

2.2

3.8

6.0

Rubber Roll Huller • Advantage § Hulling efficiency is higher. § Reduce grain breakage and loss of small broken. § The silver skin of brown rice is undamaged. § Sieving the husked products is unnecessary. • Disadvantage § The cost of replacing rubber rolls.

Husk Separation Paddy

Dehusker Brown rice, Broken, Paddy, Bran, Husk Brewer rice

Sieve

Coarse bran

Husk Aspirator

Husk

Brown rice, Broken,

Paddy

Paddy Separator

Husk Aspirator

Plansifter & Husk Aspirator

Closed Circuit Husk Aspirator

1 เกลียวลําเลียงแกลบ 2 เกลียวลําเลียงเมล็ดออน 3 เกลียวลําเลียงขาวกลอง/ ขาวเปลือก 4 แผนปรับลม 5 แผนปรับลม

Rubber roll huller with husk aspirator

Huller with Husk Aspirator

(BUHLER: DRHC/DRSC)

Paddy Separation • Removal of paddy from brown rice After husk separation, a mixture of 85-90% brown rice and 10-15% paddy is fed into the paddy separation stage. The paddy must be separated before the brown rice goes to the bran removal stage. The separated paddy is returned to the husker for dehusking.

Paddy and brown rice have different characteristics • • • •

Weight per volume of paddy is less than that of brown rice, Specific gravity of paddy is lower than that of brown rice, Dimensions of paddy are longer, wider and thicker, Coefficient of friction is different.

Types of paddy separators • Compartment (table) type • Tray type • Screen type

(BUHLER: DNTB)

Compartment-type Separator The main part of this paddy separator is the oscillating compartmentassembly where the actual separation of paddy and brown rice takes place. The compartment-assembly consists of a number of compartments in one, two, three, or sometimes four decks. The number of compartments depends on the capacity of the rice mill,

Paddy, brown rice

high low

Paddy

Brown rice

Paddy, brown rice Paddy

Brown rice

High

Paddy

a

HUSKER

Adjustment • Speed 90 - 120 strokes/min • Slope • Stroke

Lower

Brown rice

WHITENER

Tray type separator

First rubber roll huller

Second rubber roll huller

Whitening Machine • Removes bran layer to produce white rice. • Amount of bran removed (usually 8-12%)

Whitening Whitening Actions • Abrasive type High speed / Low pressure • Griding action • Impact action • Friction type Low speed / High pressure • Tearing action • Cutting action

Milled rice & bran Friction type

Abrasive type

Smooth surface

Rough surface

Large

Fine

Whitening Machine • • • •

Vertical abrasive whitening cone Horizontal abrasive whitener Horizontal friction-type whitener Bottom-Up Vertical friction-type whitener

Feeding Air

Vertical Abrasive Whitening Cone

Abrasive cone Wire mesh

Bran sweeper

Bran Rice Level adjust

Rubber brake

Number of Brakes = Cone Diameter - 2 100 Eg. Cone Diameter Number of Brakes

= 600 mm = 600/100 – 2 = 4

Peripheral Speed 13 m/s

Capacity of Cone-type Whitener (kg of brown rice / hr) Diameter

Power

1 Cone

2 Cones

3 Cones

mm

hp

Long grain

Short grain

Long grain

Short grain

Long grain

Short grain

500

4

350

420

570

680

680

800

600

7.5

550

650

890

1100

1050

1250

800

10

750

900

1230

1450

1460

1725

1000

15

1000

1200

1700

2000

1900

2300

1250

20

1350

1600

2200

2600

2600

3000

1500

25

1700

2000

2700

3200

3200

3800

Composition Whitening Cone No. 1 and No. 2 Emery grit 16 Silicium carbide grit 16 Silicium carbide grit 18

25% 50% 25%

Whitening Cone No. 3 Emery grit 18 Silicium carbide grit 18

25% 75%

Horizontal abrasive whitener

Horizontal abrasive whitener

Abrasive roll Screw

Counter Weight

Bran Aspirator

Resistance pieces

Abrasive grit #30, 36

Screen

Long grain

Short grain

900 450

% Bran

00

0

30 45 60

90

Characteristics of Abrasive Action Milling (Speed 12 - 13 m/s)

Shaping Action Factors Condition

Flat Shape Slim shape Round Shape

Peripheral Speed

Low

Medium

High

Roll Grit

Small

Medium

Large

Pressure

High

Medium

Low

Horizontal friction-type whitener

Perforated sheet Milling roll

Brown rice

P control

Milled rice

Bran

Aspirator

Screw Perforated sheet

Characteristics of Low-Speed Machine (Friction-type)

Effect of Ventilation on Low Speed Machine (Friction-type Whitener)

Bottom-Up Vertical friction-type whitener (SATAKE)

Polisher • Humidifying Rice Milling Machine • Vertical Polishing Cone – Brush type • Vertical Polishing Cone – Leather type • Horizontal Polisher – Leather type

Humidifying Rice Milling Machine

Nozzle

Bran

Water addition rate 0.3 - 0.4 %

Mold Growth on Rice from Non-Humidifying and Humidifying Milling

Humifying rice-milling machine is used in the last step of whitening system

Characteristics of Humidifying Milling Machine

Polisher

(BUHLER: DRPE)

Vertical Polishing Cone – Brush type

Vertical Polishing Cone - Leather type

Horizontal Polisher

Whitening System System

1

2

3

4

5

Long grain (Indica) 1

Abrasive

Abrasive

Abrasive

-

-

2

Abrasive

Abrasive

Abrasive

Vent Friction

-

3

Abrasive

Abrasive

Abrasive

Hu Friction

-

4

Abrasive

Abrasive

Abrasive

Vent Friction

Hu Friction

Vent Friction

Hu Friction

Short grain (Japonica) 1

Abrasive Vent Friction Vent Friction

Milling System for Indica-type variety

ABRAS

ABRAS

ABRAS

Abrasive

ABRAS ABRAS

ABRAS

FRICT

Friction

FRICT

Combined System vs Low Speed System

Combined System (High + Low speed)

1st Vertical whitening roll 2nd Vertical whitening roll 3rd Hor Humidifying friction polisher

Grading Size of brokens according to Thai Rice Standard (1997)

General standard

Japanese standard

A from B : By thickness A from C : By thickness / length B from C : By length

A from B :

By length

Bold grain A from B/C : By thickness / length B from C : By length Slender grain A from B/C : By length B from C : By length

Grading of White Rice • After polishing, white rice contains: – Head rice – Large brokens – Small brokens

• The white rice is separated using – Sifter – Length grader

Rotary Seiver

Rotary Sifter

White Rice Grading One double layers sifter and two trieurs in parallel.

One double layers sifter and three trieurs in series.

Rice Mixing • An efficient rice mill will produce: – 50% head rice (whole kernels) – 5-15% large broken and – 5-15% small broken kernels

• Depending on the country’s standards, rice grades in the market will contain from 5-25% broken kernels.

Rice Mixing Small brokens

Large brokens

Whole kernels

Variable speed motors allow the mill operator to mix the appropriate amount of whole kernels with large and small brokens.

Rice Weighing • Rice is normally sold in 50 kg sacks which must be accurately weighed and labeled.

scales

Bag filler and weighing

Color Sorter

Conveying System Scaling and Packaging System Duct Collecting System

Milling Quality Assessment

Assessment the milling quality of paddy • Sampling / dividing • Measure moisture content • Cleaning sample • Milling test • Grading

Sample divider

Scale

Sieve

Aspirator

Moisture Meter Resistance type Capacitance type

Oven for m.c. determination 130oC 1 hr 103oC 72 hr 105oC 5 hr

A, B : Weight per Volume Tester C : Volume Measuring Cylinder D : Counting plate

Rubber roll huller

Abrasive-type Whitener

Length Grader, Trieur

Friction-type Whitener

Friction-type Whitener with Length Grader

Procedures for Milling Quality Assessment Using Abrasive-type Whitener • Weigh clean paddy 250 g • De-husk paddy with the rubber roll husker • Weigh brown rice • Whiten brown rice w the abrasive type whitener for 90 s • Weigh milled rice • Separate broken from milled rice w the trieur for 90 s • Weigh head milled rice

Procedures for Milling Quality Assessment Using Friction-type Whitener (De-husking method) • Weigh clean paddy 100 g • De-husk paddy with the rubber roll husker • Weigh brown rice • Whiten brown rice w the friction-type whitener for 25 s • Weigh milled rice • Separate broken from milled rice w the length grader for 90 s • Weigh head milled rice

Procedures for Milling Quality Assessment Using Friction-type Whitener (Un-husking method) • Weigh clean paddy 100 g • Whiten paddy with the friction-type whitener for 30 s • Weigh milled rice • Separate broken from milled rice with the trieur for 90 s • Weigh head milled rice

Brown Rice Recovery =

Wt of Brown Rice x 100 % Wt of Paddy

Milled Rice Recovery =

Wt of Milled Rice x 100 % Wt of Paddy

Head Rice Recovery

Wt of Head Rice x 100 % Wt of Paddy

Milling Degree =

=

(Wt of WBR – Wt of WMR) x 100 % Wt of WBR

WBR = Weight of whole brown rice 1000 kernels WMR = Weight of whole milled rice 1000 kernels

Moisture Content =

Wt of Water x 100 Wt of Sample

After Drying Weight = (100 - Initial MC.) x Initial Weight (100 - Final MC.)

Eg. Paddy 500 t with initial MC 26% Final MC after drying is 14% Then After Drying Weight = (100 - 26) x 50000 = 430,23 t (100 - 14)

Thank you

Rice Milling System

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